The present invention relates to a scraper device for a reciprocating compressor piston rod.
Reciprocating compressors of a common type consist of one or a plurality of cylinders inside which pistons provided with reciprocating motion slide.
As far as the kinematic control mechanism for the reciprocating motion of the piston is concerned, reference is made by way of example to the kinematic mechanisms which consist of a connecting rod and crank or a connecting rod, crank and crosshead.
The second type of transfer of reciprocating motion proposed is applied to machines which do not have a direct connection between the foot of the connecting rod and the piston, but require interposition of a stem or rod.
The rod, which however is not designed to withstand transverse forces, is guided in its straight motion by a so-called crosshead, which forces the center of the articulation between the rod and the foot of the connecting rod to move along the axis of the cylinder.
The crosshead consists of a body provided with a pin for articulation with the connecting rod, and runners, wherein the axis of the pin is at right-angles to the axis of the rod and of the runners, i.e. arranged in the shape of a cross.
In this type of compressor the rod, which is fitted between the crosshead and the piston, is thus in an intermediate position between the units of the crank mechanism which transform the motion from rotary to straight, i.e. shaft, connecting rod, bearings and the like, and the compression chamber, which is formed by a cylinder and piston.
The outer structure or casing of the reciprocating compressors has a cover which separates the crank mechanism from the compression chamber through which the rod passes, and is provided with a plurality of scraper rings fitted onto the rod itself.
A common assembly configuration consists of use of a first roughing ring and a plurality of finishing rings, disposed downstream from the roughing ring relative to the area of lubrication.
The function of the scraper rings accommodated in the separation cover of the reciprocating compressors is to prevent the lubrication oil of the crank mechanism (shaft, connecting rod, bearings and the like) from escaping from the casing towards the cylinder, since it is drawn by the piston rod.
Inefficiency of the scraper ring can result in a continual loss of oil and thus cause progressive emptying of the oil from the crank mechanism, with consequent potentially very serious damage to the compressor.
At present the problem is commonly solved by using a series of bronze or plastics scraper rings, provided with a scraper lip in order to retain the film of oil present on the surface of the rod.
These rings are produced in three or four pieces, with inclined contact surfaces, and are held together by means of a spring on the outer diameter. This design solution permits replacement of the rings with the rod fitted as required by the standards API618, which are dedicated to reciprocating process compressors.
For example a common assembly configuration consists of the use of a metal roughing scraper ring which acts initially, and is accommodated in corresponding seats provided in the inner diameter of spacer elements which constitute the cover for separation of the crank mechanism from the compression chamber of the reciprocating compressor.
In a second seat provided in the spacer element, immediately downstream from the seat which accommodates the roughing ring, the finishing rings are inserted.
However the conventional metal scraper rings have proved to have reduced efficiency in the case of rods with medium or large dimensions, for example with a diameter larger than 75 mm.
In fact, when the diameter increases, there is also an increase in the probability that the ring will have imperfections of shape, caused by the inevitable inaccuracies in mechanical processing.
The use of scraper rings produced with a large number of pieces, which is prevalent in the case of rods with a large diameter, contributes in turn towards increasing considerably the risk of encountering imperfections of shape such as to give rise to blow-by of oil.
A further disadvantage of the conventional scraper rings described consists in the fact that the rings themselves must carry out both the scraper function and the function of centering of the rod. The mechanical stresses which can be attributed to the movements of the rod in a radial direction, to which the scraper rings are exposed, contribute towards the rapid wear of the latter with consequent deterioration of the scraper function and thus the risk of detrimental blow-by of oil.
A scraper device is disclosed for a reciprocating compressor piston rod which eliminates the above-described disadvantages.
The scraper device for a reciprocating compressor piston rod uses different structural elements to carry out the scraper function and the function of centering on the piston rod.
The scraper device for a reciprocating compressor piston rod permits a saving in the maintenance costs, as well as reduced machine-stoppage times, since it is characterized by a scraper ring which is easy to replace.
The scraper device for a reciprocating compressor piston rod has low wear compared with the known type, and therefore also makes is possible to reduce substantially the costs of spare parts and maintenance inventions.
The scraper device for a reciprocating compressor rod is particularly simple and functional, and has low costs.